The present invention relates to power amplifiers, such as the type used in wireless communications, and more particularly relates to determining and implementing a predistorting adjustment of a signal prior to amplification by a power amplifier for the purpose of compensating for distortion introduced by the power amplifier.
Power amplifiers are widely used in telecommunications. For example, in wireless communication applications, power amplifiers are found in the last stage of transmitter circuits, where they are used to boost the power of a supplied signal to an acceptable level for transmission from an antenna. In addition, power amplifiers are often provided with an amplitude modulation (AM) function, which enables them to operate in accordance with non-constant envelope modulation schemes, such as those used in EDGE (Enhanced Data rates for GSM Evolution), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), and the like. Even when constant amplitude modulation schemes are being used, the AM functionality of a power amplifier may be applied for power control when the transmission is performed in time slots, such as occurs in time division multiple access (TDMA) systems, of which the Global System for Mobile communications (GSM) and EDGE are but two such examples.
One problem with using commercially available power amplifiers in applications such mobile communications is that they typically exhibit distortion. For example, consider a reference input (called AMref) that is an intermediate frequency (IF) signal representing the envelope (AM) of a radio frequency (RF) signal to be modulated in any of a number of modulation schemes. Ideally, supplying AM ref to the AM input of a power amplifier will produce an RF AM signal at the output of the power amplifier without distortion. In practice, however, there is distortion generated in the power amplifier: AMref to Phase Modulation distortion (PMdist) and AMref to AM. These types of distortion can be abbreviated as AM/PM and AM/AM, respectively.
In communications applications where information is encoded in the phase and/or amplitude of the signal, the added distortion can cause serious errors to occur when the receiver attempts to extract the intended information. To avoid this undesirable result, it is possible to adjust the signal prior to its amplification in a way that will cancel out the distortion known to be added by the power amplifier. This is called “predistorting” the signal, and the amount of amplitude and phase adjustment that needs to be applied for this purpose is called “predistortion.”
It is possible to build a power amplifier having built-in predistortion, but since standard “off the shelf” components from different power amplifier vendors don't have this feature, the distortion usually has to be accounted/compensated for in other parts of the communication equipment.
It is possible to introduce predistortion in different ways to compensate for the distortion in the power amplifiers. An important aspect in this regard is that different samples of the same nominal power amplifier component will exhibit different AM/PM and AM/AM characteristics. Furthermore, the power amplifier-modulator will have varying AM/PM and AM/AM distortion over wide frequency bands. This means that different predistortion compensation is required for different power amplifiers and frequencies.
During manufacturing/production of a communications device, such as a mobile station (MS), different kinds of calibrations are made. It is desirable to have a calibration method for finding the power amplifier distortion and for applying the compensation (predistortion) needed. One way of finding the best predistortion values is to optimize the values by comparing a measured spectrum performance to an ideal spectrum performance. This type of analysis can be very time consuming though and a fast and accurate solution would be much more desirable. The problem is made even more difficult by the fact that the measurement data is also expected to be noisy, which slows the measurement process even further.
Most conventional predistortion algorithms are based on measurements of different figures of quality, such as Error Vector Modulation (EVM) and/or “spectrum due to modulation.” Consequently, an iterative process is then required for finding a predistortion that gives sufficiently good results. However, having to perform so many different types of measurements and then performing the necessary iterations for arriving at the acceptable results is very time consuming.
It is therefore desirable to have a calibration system for use in production of communication equipment that helps to 1) get excellent/robust performance of the communication equipment and 2) save time and money.